In recent years, a water-absorbent resin is widely used in sanitary goods such as disposable diapers, sanitary napkins, etc., for the purpose of absorbing and holding body fluid such as urine, blood, etc., to prevent clothes from being contaminated.
Examples of the conventionally known water-absorbent resins may be but are not limited to:
a partially neutralized crosslinked polymer of polyacrylic acid (Japanese Unexamined Patent Publication No. 84304/1980 (Tokukaisho 55-84304), Japanese Unexamined Patent Publication No. 108407/1980 (Tokukaisho 55-108407) and Japanese Unexamined Patent Publication No. 133413/1980 (Tokukaisho 55-133413)); PA1 a hydrolyzed graft polymer of starch-acrylonitrile (Japanese Examined Patent Publication No. 43995/1971 (Tokukosho 46-43995)); PA1 a neutralized graft polymer of starch-acrylic acid (Japanese Unexamined Patent Publication No. 125468/1976 (Tokukaisho 51-125468)); PA1 a saponified copolymer of vinyl acetate-acrylic ester (Japanese Unexamined Patent Publication No. 14689/1977 (Tokukaisho 52-14689)); PA1 cross-linked carboxymethyl cellulose (U.S. Pat. No. 4,650,716 and U.S. Pat. No. 4,689,408); PA1 a hydrolyzed copolymer of acrylonitrile or of acrylamide, or a cross-linked copolymer of both (Japanese Unexamined Patent Publication No. 15959/1978 (Tokukaisho 53-15959)); PA1 a crosslinked polymer of cationic monomer (Japanese Unexamined Patent Publication No. 154709/1983 (Tokukaisho 58-154709) and Japanese Unexamined Patent Publication No. 154710/1983 (Tokukaisho 58-154710)); PA1 cross-linked isobutylene-maleic anhydrous copolymers (U.S. Pat. No. 4,389,513); and PA1 cross-linked copolymers of 2-acryl-amide-2-methylpropanesulfonic acid with acrylic acid (EP068189). PA1 adding at least one member selected from the group consisting of a water-soluble surface active agent and a water-soluble polymer to dried water-absorbent resin powders of an irregular crushed shape having a carboxyl group, whose surface regions are crosslinked, in a sufficient amount for increasing an absorbing rate (g/g/sec) of the water-absorbent resin powders defined based on 28 times swelling time with artificial urine above an absorbing rate of the surface crosslinked water-absorbent resin powders, said water-absorbent resin powders having an absorbency under pressure based on the physiologic saline solution under load of 50 g/cm.sup.2 increased to at least 20 g/g. PA1 ketones, such as acetone; PA1 ethers, such as dioxane, tetrahydrofuran, alkoxypolyethylene glycol; PA1 amides, such as N,N-dimethylformamide; PA1 sulfoxides, such as dimethylsulfoxide; etc. PA1 anionically unsaturated monomers such as methacrylic acid, maleic acid, .beta.-acryloyloxy propionic acid, vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamide-2-methyl propanesulfonic acid, 2-(meth)acryloyl ethanesulfonic acid, and 2-(meth)acryloyl propanesulfonic acid and salts thereof; PA1 nonionic hydrophilic group-containing unsaturated monomers such as acrylamide, methacrylamide, N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylate, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth) acrylamide, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethyleneglycol mono(meth)acrylate, vinylpyridine, N-vinyl pyrrolidone, N-acryloylpiperidine, and N-acryloyl pyrrolidine; and PA1 cationically unsaturated monomers such as N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethyl aminopropyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and quaternary salts thereof. PA1 carbonates such as sodium (hydrogen)carbonate, ammonium (hydrogen)carbonate, potassium (hydrogen)carbonate, magnesium carbonate, carbon dioxide, ethylene carbonate, etc.; PA1 water-soluble azo compounds such as 2,2'-azobis(2-methylpropionamizine) dihydrochloride, 2,2'-azobis(2-(2-imidazoline-2-il)propane) dihydrochloride, 2,2'-azobis [2-methyl-N-(2-hydroxyethyl)-propionamide], etc.; and PA1 water uniformly dispersed azo compounds such as 2,2'-azobis(2-methylpropioneamizine)diacrylate, etc. PA1 glycidyl ethers such as ethylene glycol diglycidyl ether, polyethylene diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, etc.; PA1 glycidyl compounds such as glycidol, .gamma.-glycidoxypropyltrimethoxysilane, etc.; PA1 epihalohydrins such as epichlorohydrin, epibromohydrin, etc.; PA1 phosphonic acid glycidyl ethers such as methyl phosphonic acid diglycidyl ether, n-propyl phosphonic acid diglycidyl ether, etc.; PA1 cyclic epoxy compounds such as 3,4-epoxycyclohexane carboxylic acid-3',4'-epoxycyclohexyl ester (product name: Celoxide R 2021, DAICEL Chemical Industries LTD), and the like. PA1 polyhydroxy alcohol compounds such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol; propylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylol propane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, sorbitol, etc.; PA1 polyamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylene pentamine, pentaethylenehexamine, polyamidepolyamine, polyethyleneimine, etc., and condensation products of these polyamine compounds and haloepoxy compounds; PA1 polyisocyanate compounds such as 2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.; PA1 polyoxazoline compounds such as 1,2-ethylene bisoxazoline, etc.; PA1 a silane coupling agent such as .gamma.-glycidoxypropyltrimethoxysilane, .gamma.-aminopropyltrimethoxy silane, etc.; PA1 alkylene carbonate compounds such as 1,3-dioxolane-2-one, 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, 1,3-dioxopane-2-one, etc.; and PA1 polyvalent metallic compounds such as hydroxides and chlorides of metals such as zinc, calcium, magnesium, aluminum, iron, zirconium, etc. Only one kind of the above-listed crosslinking agent may be adopted, or two or more kinds thereof may be suitably mixed and adopted. PA1 lower alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol; PA1 ketones, such as acetone; PA1 ethers, such as dioxane, alkoxy(poly)ethylene glycol, tetrahydrofuran, etc.,; PA1 amides, such as N,N-dimethylformamide, etc.; and PA1 sulfoxides, such as dimethylsulfoxide, etc. PA1 1) The water-absorbent resin powders are reacted with the nucleophilic reagent on contact therewith in a form of gas; PA1 2) The nucleophilic reagent is mixed with the water-absorbent resin powders to be reacted therewith; PA1 3) A solution containing the nucleophilic reagent is mixed with the water-absorbent resin powders to be reacted therewith; or PA1 4) Water-absorbent resin powders are brought into contact with the solution containing the nucleophilic reagent to be reacted therewith, etc.
The properties which the water-absorbent resins are desired to possess include, for example, high absorbency and high absorbing rate to be manifested on contact with aqueous liquids, liquid permeability, high strength exhibited by the gel swollen with liquid, and ability to aspirate water from the substrate impregnated with aqueous liquid, less residue of monomer (U.S. Pat. No. 4,794,166).
These properties are not necessarily correlated positively to one another, and a problem arises, for example, in that such properties as the liquid permeability, the gel strength, and the absorbing rate are lowered in proportion as the absorbency is heightened.
As a means to improve the various water-absorbent properties of the water-absorbent resin in finely balanced levels, the technique of cross-linking the surface regions of the water-absorbent resin has been known. Various methods have been proposed concerning the technique.
For example, a method using polyhydric alcohols (Japanese Unexamined Patent Application No. 108233/1983 (Tokukaisho 58-108233), and Japanese Unexamined Patent Application No. 16903/1986 (Tokukaisho 61-16903)), a method using polyglycidyl compounds, poly aziridine compounds, polyamine compounds, and polyisocyanate compounds (Japanese Unexamined Patent Application No. 189103/1984 (Tokukaisho 59-189103) and (U.S. Pat. No. 4,666,893)), methods using glyoxal (Japanese Unexamined Patent Application No. 117393/1977 (Tokukaisho 52-117393)) , methods using polyvalent metals (Japanese Unexamined Patent Application No. 136588/1976 (Tokukaisho 51-136588), Japanese Unexamined Patent Application No. 257235/1986 (Tokukaisho 61-257235) and Japanese Unexamined Patent Application No. 7745/1987 (Tokukaisho 62-7745)), methods using a silane coupling agent (Japanese Unexamined Patent Application No. 211305/1986 (Tokukaisho 61-211305), Japanese Unexamined Patent Application No. 252212/1986 (Tokukaisho 61-252212) and Japanese Unexamined Patent Application No. 264006/1986 (Tokukaisho 61-264006)), a method using a monoepoxy compound (Japanese Unexamined Patent Application No. 87638/1992 (Tokukaihei 4-87638)), a method using a polymeric compound having an epoxy group (U.S. Pat. No. 4,758,617), a method using an epoxy compound and a hydroxy compound (Japanese Unexamined Patent Application No. 132103/1990 (Tokukaihei 2-132103)), and a method using an alkylene carbonate (DE-4020780) have been known.
Other than the described methods, methods requiring the presence of an inorganic inactive powder (Japanese Unexamined Patent Application No. 163956/1985 (Tokukaisho 60-163956) and Japanese Unexamined Patent Application No. 255814/1985 (Tokukaisho 60-255814)), a method requiring the presence of a dihydric alcohol (Japanese Unexamined Patent Application No. 292004/1989 (Tokukaihei 1-292004)), a method requiring the presence of water and an ether compound (Japanese Unexamined Patent Application No. 153903/1990 (Tokukaihei 2-153903)), a method requiring the presence of a water-soluble polymer (Japanese Unexamined Patent Application No. 126730/1991 (Tokukaihei 3-126730)), and a method requiring the presence of the alkylene oxide additive of a monohydric alcohol, an organic acid salt, lactam, etc. (EP-0555692, and U.S. Pat. No. 5,322,896), a method of mixing a surface crosslinking agent with heated water-absorbent resin in which not less than 90 percent of particles have a size of not less than 250 .mu.m (Japanese Unexamined Patent Application No. 224204/1995 (Tokukaihei 7-224204)) and a method of mixing a reducing agent with a surface crosslinking agent (U.S. Pat. No. 5,382,610) have been known.
Also, to attain improved properties of surface region crosslinked particles, a method of granulating the particles with an aqueous solution (U.S. Pat. No. 5,369,148), and a method of adding a cationic polymer having a molecular weight of not less than 2,000 in order to be fixed with the fiber material (U.S. Pat. No. 5,3282,610) have been known.
Further, EP-668080 published on Aug. 23, 1995 discloses a method in which a surface crosslinkage is carried out by adding organic acid/inorganic acid/polyamino acid.
The described methods permit some improvements in a balance of various properties of the water-absorbent resin, yet further improvements are needed to reach a desirable level. This has led to the need for further developments to attain improved properties of the water-absorbent resin.
In recent years, such a water-absorbent resin that excels in basic water-absorbent properties under pressure, especially in absorbency under pressure, while maintaining as high absorbency without pressure as the conventional water-absorbent resin has been strongly demanded.
Specifically, the demand for the water-absorbent resin which excels in absorbency under high pressure (for example, under load of 50 g/cm.sup.2) and exhibits high absorbency even under heavy load has been increasing. Here, a heavy load is defined to be a load incurred when not only a baby weighing around 10 Kg but also an adult person uses a sanitary material including a water-absorbent resin.
To meet the described needs, an improvement in crosslinkage to be applied to the surface regions of the water-absorbent resin are essential. To be specific, in order to meet the demands, it is required to increase the degree of crosslinkage of the surface regions. To do so, an amount of use of the surface crosslinking agent is increased or in order to uniformly crosslink only the surface regions, an amount of water or solvent to be added with the crosslinking agent is reduced.
However, in such cases, the crosslinking agent is likely to remain on the surface of the resin. Such problems do not occur when adopting a crosslinking agent that has low reactivity and excels in safety such as polyhydric alcohol, etc.
However, in the case of adopting the crosslinking agent of high reactivity such as epoxy compound, etc., it is likely that surface regions are crosslinked immediately, and excellent properties are likely to be obtained. On the other hand, however, the surface crosslinking agent itself is acrid to skin. Thus, when a large amount of the crosslinking agent is contained in the water-absorbent resin, a new problem is raised in its safety when applying it to sanitary materials. Namely, in the conventional water-absorbent resins, an epoxy compound remains in an order of from several tens to 1,000 ppm.
In order to reduce an amount of a residue in the resin of the crosslinking agent for crosslinking the surface regions, the surface regions of the water containing gel-like resin at a specific water content ranging from 10 to 30 percent, a method of further adding a predetermined amount of water during the process is known (Japanese Unexamined Patent Application No. 195705/1991 (Tokukaihei 3-195705)).
Such method necessitates complicated processes, and the surface crosslinking agent is penetrated to the inside of the particles because of its high water content. As a result, the absorbency under high pressure as well as without pressure is lowered to an insufficient level, and a significant effect of reducing the amount of a residue of the crosslinking agent cannot be expected. Namely, the inventors of the present invention have found that these properties are negatively correlated to one another.
In order to meet a demand for thinner diapers of higher performances, there is a tendency of reducing a fiber material such as pulp from the water-absorbent material and increasing an amount of the water-absorbent resin, i.e., increasing the density of the water-absorbent resin. However, when the density is increase in a diaper, as the water-absorbent resin has a lower absorbing rate as compared with pulp, it is required to improve the absorbing rate of the water-absorbent resin.
In order to ensure high absorbing rate of the water-absorbent resin, it is required to increase a surface area of the water-absorbent resin. However, if the absorbing rate is increased merely by reducing a particle diameter, liquid permeability is lowered. To avoid such problem, the surface area is increased without reducing the particle diameter, for example, by pulverizing the water-absorbent resin to be crushed in an irregular shape, or foaming the water-absorbent resin.
Further, a method of crosslinking the surface of foamed and porous water-absorbent resin to improve the absorbing rate and the absorbency under pressure (U.S. Pat. No. 5,399,591) is also known.
However, in the case of increasing the absorbency under high pressure by carrying out a surface crosslinkage of the foamed porous water-absorbent resin, due to its wide surface area, it is required to add the surface crosslinking agent in a still greater amount, and it is difficult to uniformly add the surface crosslinking agent to particles. As a result, it has been found that the amount of a residue of the surface crosslinking agent is increased. Namely, it has been found that an improvement in absorbing rate, which has been strongly demanded among the properties of the water-absorbent resin, and a reduction in amount of a residue of the surface crosslinking agent are negatively correlated to one another.
In order to obtain absorbency under high pressure, in general, the amount of the surface crosslinking agent is increased from the conventional method as described above. However, in such case, as the crosslinking density of the surface regions is too high, the absorbing rate is reduced on the contrary.
Namely, it is well known in the art that the absorbing rate is increased by the surface crosslinkage. However, it has been found that when surface crosslinkage is carried out to sufficiently increase the absorbency under high pressure (for example 50 g/cm.sup.2) to be durable even against heavier weight, the absorbing rate defined in the present invention is lowered compared with the water-absorbent resin before the surface crosslinkage. Namely, it has been found that an improvement in absorbency under high pressure and an improvement in absorbing rate may be negatively correlated to one another.
As described, the inventors of the present invention have raised new problems that: (1) an improvement in absorbency under high pressure durable for heavy weight and a reduction in an amount of a residue of the epoxy crosslinking agent may be negatively correlated, (2) an improvement in absorbing rate by the area/weight ratio and a reduction in an amount of a residue of the epoxy crosslinking agent may be negatively correlated and (3) an improvement in absorbency under high pressure, and an improvement in absorbing rate may be negatively correlated.
Accordingly, an object of the present invention is to provide new absorbent agent powders which exhibit an improvement in absorbency under high pressure, a reduction in an amount of a residue of the epoxy crosslinking agent and an improvement in absorbing rate, and the manufacturing method of the same.